1. Field of the Invention
The present invention relates to processes for producing synthesis gas from light hydrocarbons (e.g., natural gas) and oxygen. More particularly, the invention relates to supported nickel-rhodium based catalysts, their methods of making, and to processes employing such catalysts for net partial oxidation of light hydrocarbons (e.g., natural gas) to products comprising CO and H2.
2. Description of Related Art
Large quantities of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive.
To improve the economics of natural gas use, much research has focused on methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is reformed with water to produce carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted to hydrocarbons, for example, using the Fischer-Tropsch process to provide fuels that boil in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes.
Current industrial use of methane as a chemical feedstock proceeds by the initial conversion of methane to carbon monoxide and hydrogen by either steam reforming, which is the most widespread process, or by dry reforming. Steam reforming currently is the major process used commercially for the conversion of methane to synthesis gas, proceeding according to Equation 1.CH4+H2OCO+3H2  (1)
Although steam reforming has been practiced for over five decades, efforts to improve the energy efficiency and reduce the capital investment required for this technology continue.
The catalytic partial oxidation of hydrocarbons, e.g., natural gas or methane to syngas is also a process known in the art. While currently limited as an industrial process, partial oxidation has recently attracted much attention due to significant inherent advantages, such as the fact that significant heat is released during the process, in contrast to steam reforming processes.
In catalytic partial oxidation, natural gas is mixed with air, oxygen-enriched air, or oxygen, and introduced to a catalyst at elevated temperature and pressure. The partial oxidation of methane yields a syngas mixture with a H2:CO ratio of 2:1, as shown in Equation 2.CH4+1/2O2CO+2H2  (2)
This ratio is more useful than the H2:CO ratio from steam reforming for the downstream conversion of the syngas to chemicals such as methanol and to fuels. The partial oxidation is also exothermic, while the steam reforming reaction is strongly endothermic. Furthermore, oxidation reactions are typically much faster than reforming reactions. This allows the use of much smaller reactors for catalytic partial oxidation processes. The syngas in turn may be converted to hydrocarbon products, for example, fuels boiling in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes by processes such as the Fischer-Tropsch Synthesis.
The selectivities of catalytic partial oxidation to the desired products, carbon monoxide and hydrogen, are controlled by several factors, but one of the most important of these factors is the choice of catalyst composition. Difficulties have arisen in the prior art in making such a choice economical. Typically, catalyst compositions have included precious metals and/or rare earths. The large volumes of expensive catalysts needed by prior art catalytic partial oxidation processes have placed these processes generally outside the limits of economic justification.
For successful operation at commercial scale, the catalytic partial oxidation process must be able to achieve a high conversion of the methane feedstock at high gas hourly space velocities, and the selectivity of the process to the desired products of carbon monoxide and hydrogen must be high. Such high conversion and selectivity must be achieved without detrimental effects to the catalyst, such as the formation of carbon deposits (“coke”) on the catalyst, which severely reduces catalyst performance. Accordingly, substantial effort has been devoted in the art to the development of catalysts allowing commercial performance without coke formation.
A number of process regimes have been proposed in the art for the production of syngas via catalyzed partial oxidation reactions. For example, the process described in U.S. Pat. No. 4,877,550 employs a syngas generation process using a fluidized reaction zone. Such a process however, requires downstream separation equipment to recover entrained supported-nickel catalyst particles.
To overcome the relatively high pressure drop associated with gas flow through a fixed bed of catalyst particles, which can prevent operation at the high gas space velocities required, various structures for supporting the active catalyst in the reaction zone have been proposed. U.S. Pat. No. 5,510,056 discloses a monolithic support such as a ceramic foam or fixed catalyst bed having a specified tortuosity and number of interstitial pores that is said to allow operation at high gas space velocity. Catalysts used in that process include ruthenium, rhodium, palladium, osmium, iridium, and platinum. Data are presented for a ceramic foam supported rhodium catalyst at a rhodium loading of from 0.5-5.0 wt %.
U.S. Pat. No. 5,648,582 discloses a process for the catalytic partial oxidation of methane at space velocities of 800,000 hr−1 to 12,000,000 hr−1 on certain supported Rh, Ni or Pt catalysts. The exemplified catalysts are rhodium and platinum, at a loading of about 10 wt %, on alumina foams. The small catalyst bed used in this process is said to eliminate hot spots which are typical of relatively thick catalyst beds.
Catalysts containing Group VIII metals such as nickel or rhodium on a variety of supports are known in the art. For example, V. R. Choudhary et al. (“Oxidative Conversion of Methane to Syngas over Nickel Supported on Low Surface Area Catalyst Porous Carriers Precoated with Alkaline and Rare Earth Oxides,” J. Catalysis 172:281-293 (1997)) disclose the partial oxidation of methane to syngas at contact times of 4.8 ms (at STP) over supported nickel catalysts at 700 and 800° C. The catalysts were prepared by depositing NiO—MgO on different commercial low surface area porous catalyst carriers consisting of refractory compounds such as SiO2, Al2O3, SiC, ZrO2 and HfO2. The catalysts were also prepared by depositing NiO on the catalyst carriers with different alkaline and rare earth oxides such as MgO, CaO, SrO, BaO, Sm2O3 and Yb2O3.
U.S. Pat. No. 4,690,777 also discloses catalysts comprising Group VIII metals, such as Ni, on porous supports, for use in reforming hydrocarbons to produce CO and H2.
U.S. Pat. No. 5,149,464 discloses a method for selectively converting methane to syngas at 650° C. to 950° C. by contacting the methane/oxygen mixture with a solid catalyst comprising a supported d-Block transition metal, transition metal oxide, or a compound of the formula MxM′yOz wherein M′ is a d-Block transition metal and M is Mg, B, Al, GA, Si, Ti, Xr, Hf or a lanthanide.
U.S. Pat. No. 5,500,149 discloses various transition metals that can act as catalysts in the reaction CO2+CH4→2CO+2H2, and demonstrates how reaction conditions can affect the product yield. The partial oxidation of methane to synthesis gas using various transition metal catalysts under a range of conditions has been described by Vernon, D. F. et al. (Catalysis Letters 6:181-186 (1990)). European Pat. App. Pub. No. 640561 discloses a catalyst for the catalytic partial oxidation of hydrocarbons comprising a Group VIII metal on a refractory oxide having at least two cations.
U.S. Pat. No. 5,447,705 discloses a catalyst having a perovskite crystalline structure and the general composition: LnxA1-yByO3, wherein Ln is a lanthanide and A and B are different metals chosen from Group IVb, Vb, VIb, VIIb or VIII of the Periodic Table of the Elements.
U.S. Pat. No. 5,653,774 discloses a nickel-containing catalyst for preparing synthesis gas which are prepared by heating hydrotalcite-like compositions having the general formula:[M2+(1−X)Mx3+(OH2)]x+(Ax/nn−1)·mH2O.
A previous attempt at synthesis gas production by catalytic partial oxidation to overcome some of the disadvantages and costs typical of steam reforming is described in European Patent No. EPO 0 303 438, entitled “Production of Methanol from Hydrocarbonaceous Feedstock.”
One disadvantage of many of the existing catalytic hydrocarbon conversion methods is the need to include steam in the feed mixture to suppress coke formation on the catalyst. Another drawback of some of the existing processes is that the catalysts that are employed often result in the production of significant quantities of carbon dioxide, steam, and C2+ hydrocarbons. Also, large volumes of expensive catalyst are typically required in order to achieve satisfactory conversion of the feed gas, and to achieve the necessary level of selectivity for CO and H2 products. None of the existing processes or catalysts are capable of providing high conversion of reactant gas and high selectivity of CO and H2 reaction products. Accordingly, there is a continuing need for a process and catalyst for the catalytic partial oxidation of hydrocarbons, particularly methane, or methane containing feeds, in which the catalyst retains a higher level of activity and selectivity to carbon monoxide and hydrogen under conditions of high gas space velocity, elevated pressure and high temperature.